International researchers, analyzing results of 34 studies, found that shift work was associated with a 23 percent increased risk of heart attack among more than 2 million night, evening and rotating-shift workers, the British Medical Journal reported. Among night-shift workers only, that number rose to 41 percent. Interestingly, shift work was not associated with increased death rates. Researchers at Brigham & Women's Hospital Division of Sleep Medicine found that among workers with pre-diabetes conditions, the disease is more likely to develop in night workers than those who work days. Additionally, sleeping at "abnormal times" and not getting enough sleep leads to lower metabolism and higher spikes in blood sugar.

Teens who work "off-hour employment" before age 20 may be at risk for multiple sclerosis due to changes in their sleep patterns and disruption of their circadian rhythms, according to the Karolinska Institutet in Stockholm.

So perhaps the maligning of Edison — "the person to blame, obviously," says Dr. David Luterman, medical director and director of the Baylor Sleep Center in Dallas — now makes more sense.

"When it was dark, there was nothing to do but sleep," Luterman says. "Then the light bulb came, and then Henry Ford puts in shifts all over the place. Then radio, which was entertainment in your own home. Then the TV."

Which led, of course, to overnight jobs, to off-kilter sleep schedules and to health problems that go bump in the night.

First though, a caveat: While these shifts can lead to serious health issues, they're not a work-all-night given.

"It can mess up your life," Luterman acknowledges. "But not everybody who does shift work is affected. About 20 percent have significant problems doing it. There's no rhyme or reason who will or won't."

Every cell in our bodies, he says, has a biological rhythm. The brain has a master clock that trains us to be awake during the day and asleep at night. Thus, when light and dark are out of whack, so are we - if we're not careful.

"Simply put, the human body is designed and wired to respond to diurnal cues that have to do with nature's clock: daylight and nighttime," says Gerry Jacob. He's CEO of Wellfirst Sleep Diagnostics, whose sleep centers throughout North Texas evaluate and treat people with sleep issues.

Upending that by working at night or by alternating sleep and wake shifts can lead to dysfunction, Jacob says. For instance, it throws off production of melatonin - the hormone responsible for rest.

As far as causing obesity and other problems, Luterman says he is unsure of the correlation. There is one, he says, "but I don't know the physiology of it.

"Hormones, insulin, things like that are secreted in diurnal variation," Luterman says. "When you do shift work, you're opposing some of your normal hormone stasis equilibrium."

One problem is that people get their nights and days mixed up, he says.

"They work at night, but try to revert back to daytime on their days off. If you try to reverse, you'll have a hard time doing it. You have to shift your sleep schedule."

Significant sleep problems for night workers, Luterman says, typically fall into two categories: not being able to stay awake at work or not being able to sleep during the day.

Snoozing on the job can lead to no longer having the job, or, worse, putting lives in danger.

"You have to be 150 percent when you're with patients," says Tarah Grooms, director of the emergency department at Methodist Richardson Medical Center. "You get an influx all at once, and you don't have the administrative resources during nighttime as you do during the day. You have to be on your tiptoes at all times."

She makes sure the staff stays nourished, supplying them with nutritious snacks and water.

"As long as they eat and hydrate, they're so busy they don't even think about it," says Grooms, who lives in Coppell, Texas.

She worked nights for seven years, initially to "do my time" when she first started her career, and later when her children were younger.

"I'd go home, give them breakfast, take them to school, exercise, sleep and be able to pick them up," says Grooms, who still periodically steps in to work overnight. Whether you work days or nights, "You still have the same amount of time between shifts." A lot of her staff has a hard time getting used to sleeping during the day, she says. "But then they get on a cycle and do it. A lot here have done it for 20 years. That's their life."

The pay is also better, she says. "My turnover rate in the ER is low. Very few come in and say, 'I just can't do this.'"